RE: [alfa] Tach drive on waterpump

["Roger Hambro" <rhambro@xxxxxxx>]

Biba,

Here's another data point:
About 15 years ago when I bought my 3rd hand '78 Spider the tach was not
working and I noticed too, that the tach drive spun freely within the water
pump.  At first I thought the gear(s) were worn, but upon inspection they
seemed ok and not "hour glassed" the way a worn helical gear would need to
appear if not meshing with its mate.  Still puzzled, I did some calculations
and determined that for whatever reason the centerline of the gears were too
far apart by something like 1/16" (I don't remember the exact amount.)
Anyway I removed the tach gear drive from the water pump and turned down the
aluminum bearing holder by about 1/8" dia in order to provide about 1/16"
gap (1/16" annulus if perfectly centered in the water pump bore).  The 2
mounting screw holes in the tach gear drive need to be slotted slightly as
well.  Now I was able to slide the tach gear drive within the water pump
toward the driving gear.

Here is the trick to avoid binding the gears too tight:  You will notice
that you can take the end of the tach gear drive pin and push it in and out
slightly due to some thrust slop in the tach drive bearing holder.  So now
as you move the tach gear drive toward the driving gear, push the tach gear
drive pin in and out to "feel" just a slight amount of drag.  Now you know
that the gears are meshed.  I have put about 60k miles on the car since and
the tach still works fine.

Roger Hambro
'78 Spider
Algonquin, IL



> Date: Mon, 27 Oct 2003 17:54:52 -0500
> From: "Jeff Greenfield" <alfaguy@domain.elided>
> Subject: RE: [alfa] Tach drive on waterpump
>
> Biba -
>
> There is a gear that comes out through the front cover (it is part of the
> intermediate timing gear) that drives the tach. There is a matching gear
on
> the other end of the pin that the cable engages into that meshes with the
> gear that comes through the cover.
>
> If that tach drive turns freely, then something is wrong somewhere. Keep
in
> mind that the later intermediate gears (from Alfettas on more or less) did
> not have this protruding gear as the tachs were electronic.
>
> It wouldn't surprise me if alfa used different gears/tach drives
throughout
> the years so perhaps one must use a matched pair?
>
> HTH,
>
> Jeff
>
> - -----Original Message-----
> From: owner-alfa@domain.elided [mailto:owner-alfa@domain.elided] On Behalf Of
> alfacybersite
> Sent: Monday, October 27, 2003 2:20 PM
> To: AD
> Subject: [alfa] Tach drive on waterpump
>
> Should the tach drive end rotate freely when not in use? Does oil
> pressure (or?) force the internal drive pin to turn the gear by friction
> alone? (I switched out the tach gear from an older water pump and
> installed in 2000 engine). It rotated freely in the old pump also.
>
> On customer's 1750 engine the tach cable is to firmly attached to the
> drive to remove, but can't turn the cable by hand. Checking on another
> 'loose' engine (which has sat for eons, including outdoors) its drive
> won't move - but could be frozen.
>
> Pressing two small shafts together by friction alone doesn't seem like
> an accurate way to drive a tach.
>
> Biba
> Irwindale, CA USA
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